The piezoelectric effect has been used industrially for some time. In order to increase the displacement travel, elements constructed with the multilayer technique are often offered, for example by the PHYSIK INSTRUMENTE company in Catalog 107 of 1990. They consist of a plurality of thin piezoelectric layers, placed one on top of another, with a thickness of, for example, 20, to 500 um, between each of which an electrode layer is applied for electrical contacting. In a first embodiment having a large number of layers arranged one on top of another, such elements generate a linear motion by application of a direct-current voltage, and a linear oscillation by application of an alternating-current voltage. A second embodiment having in each case two laterally placed layers utilizes the secondary piezoelectric effect and yields a flexural motion when a direct-current voltage is applied, and a flexural oscillation when an alternating-current voltage is applied. A displacement effect going beyond the maximum linear stroke or maximum flexion cannot be achieved with these elements.
A rotatory ultrasonic drive having an unlimited displacement range has been disclosed by the SHINSEI company. It is based on an annular piezoceramic which is divided into two separately activatable excitation regions, each having 8 segments. By applying two alternating-current voltages, offset 90.degree. from one another in time, to the two excitation regions, a flexural traveling wave is generated in a stator adhesively bonded to the piezo ring. This can be used, by way of a friction layer, to drive a rotor. Other improvements to traveling-wave motors operating on the principle of the SHINSEI motor have been disclosed.
The traveling-wave principle is well-suited for rotatory motions, but it is unsuitable for linear drives. Attempts to implement a linear ultrasonic traveling wave motor by cutting open the piezo ring used in the SHINSEI motor have failed. The reasons for this are, in particular, the reflections which occur at the ends of the cut-open piezo structure, which disrupt the energy flux transported with the traveling wave and thus the entire motion. Many attempts to prevent the reflections and generate a traveling wave in a finite piezoelectric beam have so far yielded no industrially exploitable solutions.
Disclosed solutions for carrying out a linear drive mechanism are based on the use of discrete individual actuators which are excited by the application of suitable voltages in such a way that an elliptical motion for a surface point results. According to a solution described in European Patent Application No. 297 574, two mutually perpendicular longitudinal oscillations are used for this purpose; U.S. Pat. No. 4,763,776, provides two mutually perpendicular flexural oscillations.
It is an object of the present invention to provide an easily handled ultrasonic motor which is suitable for linear displacement motions.